Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels but also can be produced using grains such as corn, which is a renewable resource. Further, the production of ethanol results in new sales outlets for corn, provides additional jobs, and reduces the nation's dependency on foreign oil.
Ethanol is typically produced from corn through either a wet or dry milling process. In the wet milling process, the corn kernel is separated into various components including germ, starch, protein, and fiber, resulting in several co-products. For example, separated germ may be further processed for oil recovery; starch may be saccharified and fermented for ethanol production; and protein and fiber may be used as feed material. In a dry mill process, whole corn is ground, treated with enzymes, and cooked. The resulting “mash” is treated with enzymes to further break down the starchy endosperm tissue into glucose. The converted mash is fermented and distilled, producing ethanol, carbon dioxide, and distiller's dried grains (“DDG”), which are the undissolved solid components (i.e. stillage) remaining in the fermentation tank after the broth is removed. Thus, DDGs are typically comprised of yeast and unfermented components of the corn.
The dry grind process converts corn into two products, including ethanol and distiller's grains with solubles. If sold as wet animal feed, the co-product is known as distiller's wet grains with solubles (“DWGS”). Conversely, if dried for animal feed, the co-product is known as distiller's dried grains with solubles (“DDGS”). In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kilograms (i.e. approximately 18 pounds) of DDGS in addition to the approximately 10.2 liters (i.e. approximately 2.7 gallons) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall ethanol production cost.
Within a typical ethanol production facility, current process technology requires heat be removed from the process at various rates and locations. The standard method for heat removal is with the use of cooling water. This cooling water typically forms part of a closed loop system that picks-up heat via heat exchangers and condensers and is then returned to a cooling tower where the heat is removed via evaporative cooling. The cooling water is then recirculated back to the process.
This evaporative cooling is a major loss of water in the ethanol production facility. Because the water is evaporated, the amount of water in the cooling system is reduced. Thus, in order to keep the proper amount of water in the system, additional make-up water must be introduced. Furthermore, by evaporating water, minerals in the water will concentrate. If this cycling is uncontrolled the minerals will reach levels that will cause harmful deposits within the cooling system. Therefore, a blow down system is typically employed to discard the water with high mineral concentration and replace it with even more clean make-up water. Because there is a great deal of scrutiny in the ethanol industry regarding water use, alternative means of cooling are desirable.
As stated above, heat is extracted from the ethanol production process in numerous areas such as fermenters, vent condensers, propagator coolers, etc. One other exemplary location where water cooling is used is in the distillation area. Typical distillation includes a rectifier column to obtain the highly concentrated ethanol vapors and a condenser that cools the vapors to obtain 95% pure ethanol (i.e. 190 proof ethanol). In many facilities the 190 proof ethanol is condensed directly off the top of the rectifier column typically by a water-cooled condenser. The water-cooled condenser at this point uses approximately 50% of the cooling tower's water and approximately 25% of the entire plant's water. Other facilities do not have a phase change in distillation directly, but instead run the evaporation into a condenser, which is also a significant location of water usage for the facility.
A unique aspect of the distillation stage compared to other stages in the ethanol production process is that the water-cooled condensers can run at significantly higher temperatures than the water-cooled condensers used for other areas of the plant. This is because ethanol vapors condense at high temperatures, more specifically at temperatures 78° C. or less. Other condensers in the ethanol production process require temperatures cooler than the ambient air. Furthermore the heat captured by the water cooling in the distillation stage is not very significant as to make any considerable use of it in the rest of the facility. Therefore, because of the high amount of water usage, the high condensation temperature, and the minimal recyclability of heat, the distillation stage is prime location to replace the water-cooled condensers with an alternative condenser such as an air-cooled condenser.
Although air-cooled condensers themselves are not novel and used in place of water-cooled condensers in many industries including the ethanol production industry as exemplified in U.S. Pat. Application Publication No. 2009/0166172, they could be implemented in the distillation stage to reduce the use of water and correct for some of the issues that arise from water usage. Since the temperature required during the distillation step is 78° C. or less, it is a prime location for an air-cooled condenser because the distillation condensation requires temperatures that rarely exceed ambient temperature anywhere on Earth. Other condensers in the plant may require temperatures that are less than the ambient air. Therefore an air-cooled condenser may not be appropriate there.
Therefore, what is needed is an ethanol distillation process that employs an air-cooled condenser for extracting ethanol from an ethanol vapor stream in an effective manner as to reduce the usage of water in the ethanol production facility.